The present invention relates to a method of purifying starting materials for fabricating chalcogenide glass suitable for manufacturing optical fibers having excellent infrared transmittance and weatherability.
Chalcogenide glass is well known as a material available for transmitting infrared radiation and further is excellent in weatherability and heat resistance. Especially, chalcogenide glass is considered to be promising material for manufacturing so-called infrared fibers such as an energy propagation path for CO.sub.2 laser beam (wavelength: 10.6 .mu.m) and a waveguide for a radiation pyrometer, etc.
Heretofore, there have been known various chalcogenide glass compositions, which are capable of transmitting infrared rays of wavelength not shorter than 10.6 .mu.m as well as being excellent in heat resistance and weatherability, which compositions comprises Ge-Sb-Se (T. Katsuyma, et al., Appl. Phys. Letter, 45, (1984) 925), Ge-Se-Te-Sb (Maruno and Noda, Asahi Glass Industrial Technology Promotion Association Research Report, 35, (1979) 367), Ge-Se-Te-Sb (S. Bordas, et al., Thermo Chimica Acta, 28, (1970) 387), etc.
However, these glass compositions have a problem that absorption occurs at a wavelength in the vicinity of 12.5 .mu.m which is ascribable to expansion and contraction vibration or oscillation of Ge atom as constituent element and O atom contained as impurity, resulting in that optical characteristis could not be improved. This is because molar absorptivity of this absorption in the glass is high. Further, even if the glass contains a very small amount of oxygen (several ppm), the absorption is clearly confirmed in the glass. In addition, the foot of its absorption curve extends over a wavelength region shorter than 10 .mu.m, thus disadvantageously causing transmission loss when the glass is used for manufacturing optical fibers.
It has been proposed to add Al to Ge and then melt Ge-Sb-Se or Ge-As-Se glass to reduce absorption due to oxygen impurities (A. R. Hilton, et al., J. Non-Cryst. Solids, 17, 319-338 (1975)). This method does not involve distillation of the starting materials. Further, Hilton et al. reported that use of Mg instead of Al failed to give rise to similar effects. Although oxygen-gathering effect of Al is utilized in the fabricatin of Ge-Sb-Se glass (Appl. Phys. Lett. supra) Al is added to the glass composition containing Ge and no distillation of starting materials themselves is carried out.
In view of increasing demand for higher performance of optical fibers using the chalcogenide glass, further improvements in the method of purifying starting materials for producing chalcogenide glass have been desired.